Niger's top court on Friday lifted the immunity of overthrown president Mohamed Bazoum, paving the way for a possible trial after his ouster in a July 2023 military coup.
"The court orders the lifting of Mohamed Bazoum's immunity," said Abdou Dan Galadima, president of the court, created in November by the new military regime.
The Niger authorities accuse Bazoum of treason, financing terrorism and plotting to undermine the state.
He has been held at the presidential residence with his wife Hadiza since the coup on July 26.
The couple has been forced to live in spartan conditions without a telephone or visitors apart from their doctor since October, according to their team.
"This decision clearly condones serious violations of the rights of the defence and marks the start of legal proceedings against the president, who is held illegally," a collective of lawyers representing Bazoum said in a statement sent to AFP.
One of them, Moussa Coulibaly, called the decision "the advent of the arbitrary", adding it had "extremely serious consequences".
Bazoum, 64, is accused of having spoken by telephone with French President Emmanuel Macron and US Secretary of State Antony Blinken in a bid for support "by an armed intervention" during the coup.
'Serious irregularities'
On the eve of the court's decision, Human Rights Watch denounced "unfair" proceedings and "serious irregularities".
The hearing had been postponed twice, with Bazoum's lawyers complaining of several obstacles to the right of a defence.
"We haven't been allowed to meet our client and the court refused to hear our arguments," Coulibaly said on Friday.
In December, the court of the West African bloc, ECOWAS, ordered his immediate release.
A month later, Niger pulled out of the regional grouping, which had initially imposed sanctions on Niamey after the coup, before lifting them in February.
Niger has pivoted away from its traditional international alliances since the military leaders came to power.
The country has long been a linchpin in the US and French strategy to combat jihadists in West Africa.
But Niger's new ruling generals kicked out troops from its former colonial power France before the end of last year and has turned its back on Paris.
It also similarly demanded the withdrawal of US forces which has begun and is due to be complete in September.
In the meantime, it has sought new partners such as Iran and Russia.
Niamey maintains close ties with military-led Burkina Faso and Mali and together they have created regional grouping the Alliance of Sahel States.
But Niger faces diplomatic tensions with Benin and refuses to reopen the border with its neighbour.